The Drosophila specie D. ananassae is characterized by hypermutability, meiotic male crossing over and a transposon, tom that specifically jumps to 22 sites in the genome cause dominant eye morphology mutations, Om. In order to understand how the tom element specifically jumps to Om loci and how the retrovirus-like element causes the defect in the development of the eye a particular locus, Om(ID), has been intensively studied. Over twenty mutations have been isolated and characterized at the genetic, cytogenetic and DNA level. Extensive cloning in the Om(ID) site has identified most of tom insertions; however more cloning is ongoing. Additional molecular and genetic analysis has been carried out on other Om loci and the Som locus where many spontaneous dominant suppressors have arisen.